This invention relates to staring array imaging systems comprising array devices which generate signals in response to an image from a distant scene, particularly but not exclusively for infrared imaging with arrays of, for example, cadmium mercury telluride photo-sensing elements. The invention further relates to imaging array devices.
In order to obtain large linear arrays, it is known to form such a device with a plurality of discrete arrays of the photo-sensing elements, the discrete arrays being arranged beside each other to provide a large composite array. One such arrangement comprising a linear array of 200 elements is described in, for example, IEEE Transactions on Electron Devices, Vol. ED-27, No. 1, January 1980, pages 150 to 154. Each discrete array of 50 cadmium mercury telluride elements is formed on its own sapphire substrate. Each of these detector elements is a separate body which is 75 .mu.m (micrometres) wide, and neighbouring element bodies are spaced apart by gaps of 75 .mu.m so that the centre-to-centre spacing of these elements is 150 .mu.m.
In order to maintain this same centre-to-centre spacing for the end elements facing each other from neighbouring discrete arrays, these end elements are formed at only a half of the gap spacing (i.e. 37.5 .mu.m) from the butting edges of substrates. Careful precautions are required in the fabrication of the elements to avoid performance degradation for these end elements situated close to the substrate edge.
Although the present applicants are able to form high performance end elements even closer to the butting edges of substrates or array bodies, there are situations where it is necessary or even desirable to arrange the discrete arrays in a spaced relationship. Thus, for example, very closely packed 2-dimensional arrays of elements can be formed in a common body of infrared sensitive material using the technology disclosed in published United Kingdom patent application (GB-A) 2,095,905, with no gaps between the elements. It is difficult to butt such discrete body arrays in 2 dimensions, particularly where a common connection is made to the edge of the body. Furthermore, it is usual to retain around the edge of the array body a width of the radiation-sensitive material which is larger than the centre-to-centre spacing of the elements, particularly when this spacing is very small. Thus, even if such body arrays were satisfactorily butted, the centre-to-centre spacing of the elements facing each other from neighbouring body arrays would be larger than that of the elements within these arrays.
It is often desirable to mount such a discrete array on a semiconductor substrate comprising signal processing circuitry. Although two or more body arrays may be butted together on the same circuit substrate, there is a practical manufacturing limit on the maximum area of a circuit substrate and this determines a maximum number of body arrays for a single circuit substrate. With very closely packed arrays, the area of the substrate needed to accommodate all the desired circuit elements and connections for one body array may be larger than that array, as a result of which the discrete arrays would be spaced from each other even if the circuit substrates were butted together. Furthermore it is exceedingly difficult to butt together such circuit substrates satisfactorily, particularly in 2 dimensions, while making all the required circuit connections between the discrete substrates.
In order to avoid butting array substrates or bodies, it is known from published United Kingdom patent application (GB-A) 2,000,658 to arrange linear arrays of photosensing elements 9 in a line in two separate series. The arrays are so spaced along each line as to have between adjacent arrays in the line a non sensitive, non image-forming space 5 having the same width in the direction of the line as the photosensitive length 8 of the linear array. The imaging system has two objectives in the form of projection lenses 2 and 3 arranged side by side with parallel optical axes so as to project identical images of a strip from a distant scene G onto both the lines B and B' of arrays. By a lateral shift in the arrangement of arrays in each line, it is arranged that the non image-forming spaces 5 between arrays of one line (B) for a given point of the scene viewed by one lens (2) corresponds to the photosensitive lengths 8 of the arrays of the other line (B') for the same scene point as seen by the other lens (3), and vice versa. As mentioned in GB-A 2,000,658, such an imaging system may be used in a camera for obtaining strip photographs or video line pictures taken from high altitudes, from satellites and/or in astronomy.
However, such projection lenses to avoid array-butting and to eliminate the effect of non image-forming spaces between arrays are not suggested for 2-dimensional arrays, and they do not seem to be usable with a plurality of arrays arranged in 2 dimensions to form a large 2-dimensional composite array such as desired for staring array imaging systems viewing a 2-dimensional scene. Such an extension of the teaching in GB-A 2,000,658 to a 2-dimensional arrangement currently seems to be impractical, as regards the side-by-side arrangement of projection lenses and the image cross-talk of various parts of a 2-dimensional scene becoming confused in the real images of the scene projected by the lenses onto the arrays.
Without necessitating butting of array substrates or bodies, the present invention permits 2-dimensional formation of large composite arrays of photo-sensing elements which are suitable for use in staring array imaging systems and in which the centre-to-centre spacing of the elements facing each other from neighbouring discrete arrays is, when viewed from the front, approximately equal to that of the elements in these discrete arrays. This is accomplished by providing each array with an image transfer or redirecting means forming a virtual image, particularly but not exclusively a magnifying lens.
According to one aspect of the present invention there is provided a staring array imaging system comprising an imaging array device which generates signals in response to an image from a distant scene, and an imaging objective from which the image of the distant scene is projected onto the device, wherein the device comprises a plurality of discrete arrays of photo-sensing elements, each discrete array is a 2-dimensional array of the photo-sensing elements, the plurality of discrete arrays are arranged in 2 dimensions to provide a large composite array, a corresponding plurality of image transfer means are arranged side-by-side in 2 dimensions in front of the discrete arrays to form a substantially continuous optical transmission plane through which the image projected from the objective is transmitted to the discrete arrays, each of the image transfer means being such as to form a virtual image of its respective discrete array as viewed from the objective and being so arranged in the transmission plane in relation to its discrete array that, in the composite virtual image of the composite array as viewed from the objective, the centre-to-centre spacing of the elements facing each other from neighbouring discrete arrays appears to be approximately equal to that of the elements in these discrete arrays.
The invention thus provides for such a system, an imaging array device which generates signals in response to an image from a distant scene, the device comprising a plurality of discrete arrays of photo-sensing elements, the discrete arrays being arranged to provide a large composite array, characterised in that image transfer means are located in front of the arrangement of discrete arrays to form a substantially continuous optical transmission plane through which the image from the scene is transmitted to the composite array of the elements, the image transfer means being so arranged in relation to each discrete array that, in the composite virtual image of the composite array as seen from in front of the image transfer means, the centre-to-centre spacing of the elements facing each other from neighbouring discrete arrays appears to be approximately equal to that of the elements in these discrete arrays.
According to another aspect of the invention, there is provided an imaging array device comprising a plurality of discrete arrays of photo-sensing elements, the discrete arrays being arranged to provide a large composite array, characterised in that a corresponding plurality of lenses is side-by-side to form a substantially continuous optical transmission plane which is located in front of the arrangement of discrete arrays and through which the image from the scene is transmitted to the composite array of the elements, each lens having such a magnification in relation to its discrete array that, in the composite virtual image of the composite array as seen from in front of the arrangement of lenses, the centre-to-centre spacing of the elements facing each other from neighbouring discrete arrays appears to be approximately equal to the magnified centre-to-centre spacing of the elements in these discrete arrays.
It should be noted that the segmentation of an optical plate to form individual magnifying lenses one for each of the individual photosensing elements of an array is known from published United Kingdom patent specification (GB-A)- 1,525,562, both for linear arrays and for 2dimensional arrays. By having one magnifying lens per photosensitive element, the area of that element "looks", to the incident radiation, to be larger than it actually is. The lens redirects the radiation from one scene pixel so that it all falls on the sensitive area of the element without any significant loss to the non-sensitive areas between the elements of the array. However, such an arrangement of magnifying lenses does not assist in obviating problems in assembling a plurality of discrete arrays to form a large composite array which "looks", to the incident radiation, as though the centre-to-centre spacing of the elements facing each from neighbouring discrete arrays is approximately equal to the magnified centre-to-centre spacing of the elements in these discrete arrays. Such a situation is achieved in accordance with the present invention by providing each discrete array with its own magnifying lens or other image transfer means in accordance with the invention.
Arrangements in accordance with the invention permit the assembly of spaced discrete arrays to form a very large composite array on which an image from the scene can be projected without encountering significant difference or discontinuity in the centre-to-centre spacing of the photo-sensing elements at the transition from one discrete array to the next discrete array. Thus, for example, when all the discrete arrays have the same centre-to-centre spacing of their elements, a large composite array having the same resolution throughout can be obtained. However, the composite array may have different resolution in different areas. Since the arrangement of lenses or other image transfer means forms a continuous transmission plane for the image of the scene, all of the scene being imaged can be viewed by the composite array. Depending on constructional features of the equipment comprising the device, the lens arrangement or other image transfer arrangement may be supported separate from or mounted on bodies or substrates comprising the discrete arrays; in the latter case, efficient image transmission can be obtained with optical immersion of the discrete arrays, and the arrays may support the image transfer means or the image transfer means may support the discrete arrays. As will be described later, various adjustments can be included in the fabrication and arrangement of the image transfer means, the discrete arrays and their elements to compensate at least partially for various image distortions which may occur in some imaging systems. For this purpose, the use and adjustment of the magnification of lenses as the image transfer means is particularly beneficial.
Such magnifying lenses or other image transfer means forming a virtual image of each array may be used with linear arrays of photo-sensing elements, but the invention is of particular advantage for forming large area composite arrays in two dimensions. Thus, preferably, each discrete array is a 2-dimensional array of photo-sensing elements, and the arrangement of both the discrete arrays and the lenses or other image transfer means may extend in 2 dimensions. The resulting device may be a staring array.
Thus, the present invention permits a staring array imaging system to be designed, comprising a 2-dimensional array device in accordance with the invention, and comprising an imaging objective from which an image of the distant scene is projected onto the device, the continuous optical transmission plane formed by said arrangement of lenses or other image transfer means being so located in relation to the image plane of the objective as to provide the focussed image from the scene on the photo-sensing elements.
Depending on its characteristics and angle of view, the objective may have an image plane which is curved. In this case the discrete arrays and/or their corresponding lenses or other image transfer means can be assembled and arranged so as to reduce or even to avoid image distortion. Thus, the continuous optical transmission plane formed by the arrangement of lenses or other image transfer means associated with the discrete arrays may be bent or curved similar to the curved image plane of the objective.
As will be described later, intermediate centre-to-centre spacing of elements may be provided in the fabrication of the elements approaching the facing edges of discrete arrays of different resolution. Even when composite arrays of the same resolution are formed, small adjustments may be desirable in the centre-to-centre spacing of particular elements as fabricated in the discrete arrays. Thus, for example, the elements within a discrete array may be arranged in a pattern which compensates for optical distortion of the image, for example by the objective or by the image transfer means associated with that discrete array.
Furthermore, as described later, the discrete arrays may be fabricated with more elements than are separately visible adjacent the array edges in any one angular view of the virtual image of that array as magnified. The use of such a large magnification as to superimpose, in the virtual image, the images of edge-adjacent elements of neighbouring arrays can be particularly beneficial for wide angle rays from the scene, especially those incident on peripheral arrays away from the centre of a very large composite array.
These and other features in accordance with the present invention will now be illustrated further in the description of specific embodiments, given by way of example, with reference to the accompanying diagrammatic drawings in which: